Data is frequently protected by making a backup of the data. A full backup copies an entire data set. A cumulative incremental backup copies data modified since the most recent full backup. A differential incremental backup copies data modified since the most recent backup of any kind. Typically, the storage media for backups is tape. However, other media such as disks can be used. If data is lost, it may be restored using the most recent full backup and appropriate incremental backups that follow it. If data is corrupted or it is desired to obtain previously deleted data, a full backup and appropriate incremental backups may be used to restore data to its state at some point-in-time.
To restore data from a full backup and one or more incremental backups, the full backup is restored and then the incremental backups are restored in the order that they were made. For example, consider a backup technique that makes a full backup on Sunday, differential incremental backups on Monday, Tuesday, Thursday, and Friday, and a cumulative incremental backup on Wednesday. If data is lost on Friday before the Friday incremental backup, the Sunday full backup is restored followed by the Wednesday cumulative incremental backup and then the Thursday differential incremental backup. This may take a considerable amount of time. It may also include restoring particular data blocks or files multiple times if they are opened and saved often. While the need to restore backup data is rare, when the need does arise there is often a desire to accomplish the restoration quickly.
What is needed is a method of efficiently restoring backup data.